Sensing behaviors of lipophilic calix[4]arene phosphonate based Langmuir-Blodgett thin films for detection of volatile organic vapors

This study concentrates on the lower rim effect of calix[4]arenes based lipophilic phosphonate (CTPO4 and CPO4) with and without tert-butyl group Langmuir-Blodgett (LB) thin films in the volatile organic vapors (VOCs) sensing applications. Surface plasmon resonance (SPR) technique was employed to determine optical parameters and to investigate VOCs sensor properties of thin films. The thicknesses of 3 layered CTPO4 and CPO4 thin films were estimated as 1.9 +/- 0.02 and 2.6 +/- 0.02 nm. Atomic force microscope (AFM) and contact angle (CA) analysis were taken for the morphological study. Both thin film surfaces showed a uniform distribution, the surface roughness of CTPO4 thin film (23.04 nm) was found to be higher than the CPO4 thin film (10.53 nm). The equilibrium contact angle values of 58.96 degrees +/- 0.43 degrees for CTPO4 and 44.23 degrees +/- 2.25 degrees for the CPO4 thin film surface were determined. CTPO4 and CPO4 LB thin films were exposed to various VOCs gas in real time. Ficks' second law of diffusion was adopted to explain the sorption process of VOCs into the thin films, and the diffusion is interpreted as a two-region interaction. The diffusion coefficients were determined between 2.7 x 10- 22-63.19 x 10-22 cm2 s- 1 and 2.7 x 10- 24-63.19 x 10-24 cm2 s- 1 for the first and second region of the interaction respectively. The responses of CTPO4 are higher than CPO4 due to the upper rim tert-butyl groups onto CTPO4 molecule. Both films were highly affiliated by the chlorinated VOCs compared with the aromatic compounds. The presence of tert-butyl group has improved sensing properties and these new calix[4]arenes are good candidates for sensor applications.